The traction, or propelling force, provided by the drive wheels of an automotive vehicle is determined by the torque at the output of a transmission (output torque) or by the output power of the transmission. The output torque is determined by two variables, viz.: the transmission ratio and the engine torque.
In the present context, the value of the transmission ratio is related to the value of the transmitted torque, it being understood that at a transmission efficiency below 1, the transmitted torque differs from the transmission ratio of the engine speed. The relevant engine torque is the torque generated by the engine minus the power consumed by accessory loads. A vehicle with a stepless i.e. continuously variable transmission (CVT) of the kind referred to, can provide an output torque defined on the basis of the traction desired by the driver of the vehicle by an infinite number of combinations of engine torques and transmission ratios.
It has been suggested to coordinate the transmission output torque directly, a pedal value, i.e. the position of the accelerator pedal; however, this results in excessive lost motion of the accelerator pedal and in poor control at high speeds. It is also known to derive a desired transmission output torque value from the position of the accelerator pedal and the engine speed. This, however, leads to an undesirable feedback when the transmission ratio is changed.
In vehicles equipped with continuously variable transmissions (CVT) and conventional throttle valve controls, where the throttle is regulated solely as a function of the accelerator pedal position, the transmission ratio is coordinated with the position of the accelerator pedal and, hence, of the throttle valve, in a manner which, results in a continuously increasing transmission, i.e. reduced transmission ratio, with increases in accelerator and throttle valve values. This improves driving performance, but the full benefits of a range of fuel efficiency at low transmission ratios and high engine torques cannot be realized.